Although Ca.sup.2+ is a well established intracellular messenger, there are still many unanswered questions concerning the kinetics and spatial localization of its effects. Because of the importance of calcium as an intracellular messenger, a variety of methods have been developed for measuring intracellular Ca.sup.2+ levels. The most successful of these use dye-linked chelators which change absorbance or fluorescence upon binding Ca.sup.2+ ions. Existing indicators for physiological cations such as Ca.sup.2+, as well as, Mg.sup.2+, H.sup.+, and Na.sup.+ change their optical properties reversibly in response to changes in cation concentrations. See, for example, U.S. Pat. No. 4,603,209, and EP 314,480. Chelating compounds have been developed that facilitate changing the intracellular calcium concentration. See, for example, U.S. Pat. Nos. 4,689,432, 4,806,604, and 5,141,627; and EP 177,202, in which a photochemical reaction causes the release or uptake of Ca.sup.2+ and thereby alters the ambient Ca.sup.2+.